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Pond 016 Slide Remediation Plan <br />TR-73 Response to Adequacy Comments-2"d Round <br />Page 2 <br />Response: NWCC, under contract to SCC, will provide compaction testing services during <br />construction of fills in Pond 016A. The purpose of their involvement is to ensure that <br />compaction testing will be conducted in accordance with standard materials engineering <br />practices from the laboratory to the field to the certified report following completion of the <br />project. Material samples will be collected and sent to NWCC's laboratory for maximum dry <br />density and optimum moisture content (Standard Proctor) testing in accordance with ASTM <br />D698/AASHTO T99 to develop the appropriate curves. <br />3. In response to the Division's comment regarding level of compaction, SCC refers to the <br />permit [Tab 13, page 33], which requires a minimum of 90 percent of Standard Proctor <br />density. While this may be the approved permit condition here, the Division strongly <br />recommends that a minimum of 95% MDD be achieved, in accordance with standard <br />engineering practice for fine-grained, cohesive soils. The Division is aware of other mines <br />that require 95% MDD. <br />Compaction in cohesive soils is best achieved by the use of a sheepsfoot roller. Please revise <br />the language of Attachment 20-D to include a statement that a sheepsfoot roller will be used, <br />if necessary, to achieve the required level of compaction. <br />In response to the Division's comment regarding moisture content, SCC replies that NWCC <br />believes that acceptable compaction can be obtained at OMC +/- 4%. Again, the Division <br />strongly recommends that a narrower range of moisture contents be targeted. This will be <br />especially true if the 95% MDD compaction is to be achieved. A range of OMC +/- 2% <br />would be preferred, and is standard engineering practice. When fine-grained, cohesive soils <br />are placed in a drier than optimum condition, with reduced levels of compaction, moisture <br />tends to be drawn in from adjacent areas. This moisture can contribute to volume changes, <br />piping, and potential instability. <br />With respect to the lift thickness, neither the Rules (4.03.2(3)0) nor the PAP (Tab 13, page <br />33) allow for lift thicknesses to be increased beyond 12 inches, unless sufficient large size <br />rock is present. Please revise the proposed lift thickness to comply with the rules and the <br />language of the PAP. <br />Response: Attachment 20-D has been revised. <br />SCC will compact to 95% MDD in 12 inch lifts with a sheepsfoot roller (which is what SCC <br />already uses for construction and compaction). <br />4. Response previously accepted <br />5. Response previously accepted <br />6 Response previously accepted <br />Seneca Coal Company • P.O. Box 670 • Hayden, Colorado 81639 <br />Telephone (970) 276-5219 9 FAX (970) 276-5222 <br />